The development of portable telecommunication equipments is remarkable in these years, and especially cellular phones are being made smaller and lighter quickly. In addition to the cellular phones, electronic apparatuses such as handy cameras (portable video cameras etc.), cordless telephones, laptop PCs (notebook PCs) and the like are also being made smaller and lighter. They have a secondary battery provided within an electronic apparatus main body so as to be usable without connection to a wall outlet. Thus, their portability and usability are enhanced. But, the capacity of the secondary battery is limited, and it is required to recharge once per several days to several weeks.
Recharging methods include a contact charging method and a non-contact charging method. The contact charging method performs charging with an electrode of a power receiving device directly contacted with an electrode of a power feeding device. The contact charging method is generally used because its apparatus has a simple structure. But, the weight of the electronic apparatus has been reduced with the downsizing and weight reduction of the electronic apparatus in these years, and the contact pressure between the electrode of the power receiving device and the electrode of the power feeding device becomes insufficient, causing a problem of defective charging. Since the secondary battery is weak against heat, it is necessary to design a circuit so that the secondary battery is not overdischarged or overcharged in order to prevent the battery from increasing its temperature.
Under the circumstances described above, the application of the non-contact charging method is being studied. Conventional non-contact charging methods mainly use a nickel-hydrogen battery as a secondary battery and therefore many of them have a disadvantage that their charging time is about eight hours. Meanwhile, since a high capacity secondary battery such as an Li ion secondary battery has appeared, the application of the non-contact charging method is being studied even for electronic apparatuses such as cellular phones, notebook PCs and the like which have relatively large electric power consumption and require fast charging.
The non-contact charging method provides a coil to both the power receiving device and the power feeding device and performs charging by electromagnetic induction. The non-contact charging method does not need a contact pressure between the electrodes taken into consideration. Therefore, a charging voltage can be fed stably without being influenced by the contact state of the electrodes. As a coil for the non-contact charger, a structure that a coil is wound around a ferrite core is known (see References 1, 2). A structure that a coil is mounted on a resin substrate having a ferrite powder and an amorphous powder mixed is also known (see Reference 3). But, the ferrite becomes brittle and has poor impact resistance when it is fabricated into a thin form, and the power receiving device tends to have a malfunction if the equipment is dropped.
To provide a thin power receiving portion in correspondence with the thinning of the equipment, there is adopted a planar coil that is formed with a metal powder paste printed in a spiral form on a substrate. There is proposed a configuration that coupling between a planar coil (primary coil) of a power transmission side and a planar coil (secondary coil) of a power receiving side is reinforced by a magnetic body (see References 4 to 6). The magnetic body (magnetic sheet) is used as a core material for enhancing the coupling between the primary coil and the secondary coil. When a power transmission speed becomes high, it becomes necessary to consider not only the coupling between the coils but also heat generation of peripheral parts.
Namely, in a case where the planar coil is used, magnetic flux passing through the planar coil is interlinked with the substrate and the like within the equipment, so that heat is produced within the device because of an eddy current generated by the electromagnetic induction. Therefore, there is a problem that high electric power cannot be transmitted, and a charging time becomes long as a result. For example, when the power transmission speed is increased without regard to heat generation, it is worried that carbon dioxide is produced within the Li ion secondary battery to expand and rupture it. Therefore, charging of a cellular phone by the non-contact charger takes about 130% longer than the charging time when charged by the contact charger.
The Li ion secondary battery used for cellular phones, digital cameras, portable game machines, portable AV devices and the like has a charging capacity per unit time increased to five times or more in comparison with a conventional nickel-hydrogen battery. Therefore, when it is intended to increase a power transmission speed by the non-contact charging method, a problem of heat generation due to the eddy current is nonnegligible. Thus, a power receiving device applying a conventional non-contact charging method is insufficient to take measures against the eddy current generated due to the electromagnetic induction and against heat generation due to the eddy current. The generation of the eddy current leads to the generation of noise and also becomes a factor for lowering the charging efficiency.
[Reference 1] JP-A 11-265814(KOKAI)
[Reference 2] JP-A 2000-023393(KOKAI)
[Reference 3] JP-A 09-190938(KOKAI)
[Reference 4] JP-U 58-080753(Japanese Utility Model Publication)
[Reference 5] JP-A 04-122007(KOKAI)
[Reference 6] JP-A 08-148360(KOKAI)